Grep --byte-offset not returning the offset (Grep version 2.5.1)

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Hi,
I am trying to get the position of a repeated string in a line using
Code:
grep -b -o "pattern"
In my server I am using GNU grep version 2.14 and the code is working fine. However when I am deploying the same code in a different server which is using GNU grep version 2.5.1 the code is not working properly. Even though the byte offset option is available there. Any idea how to solve it.
Example:
Code:
export string="abc cat mat rat cat bat cat fat rat tat tat cat"
echo $string|grep -b -o "cat"
Expected output (and supported in grep 2.14):
Code:
(HowTos)